The present invention relates to fuel cells and particularly to solid polymer electrolyte fuel cells incorporating a proton exchange membrane. More particularly, the present invention relates to a corrugated flow field plate assembly for a fuel cell and a means for interconnecting fluid flow channels therein.
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Conventional proton exchange membrane (xe2x80x9cPEMxe2x80x9d) fuel cells generally employ a planar, layered structure known as a membrane, electrode assembly (xe2x80x9cMEAxe2x80x9d), comprising a solid polymer electrolyte or ion exchange membrane, which is neither electrically conductive nor porous, disposed between an anode electrode layer and a cathode electrode layer. The electrode layers are typically comprised of porous, electrically conductive sheets with electrocatalyst particles at each membrane-electrode interface to promote the desired electrochemical reaction.
In conventional fuel cells, the MEA is interposed between two rigid, planar, substantially fluid-impermeable, electrically conductive plates, commonly referred to as separator plates. The plate in contact with the anode electrode layer is referred to as the anode plate and the plate in contact with the cathode electrode layer is referred to as the cathode plate. The separator plates (1) serve as current collectors, (2) provide structural support for the MEA, and (3) typically provide reactant channels for directing the fuel and oxidant to the anode and cathode electrode layers, respectively, and for removing products, such as water, formed during operation of the fuel cell. Fuel channels and oxidant channels are typically formed in the separator plates, and such plates are then normally referred to as fluid flow field plates. Herein, xe2x80x9cfluidxe2x80x9d shall include both gases and liquids; although the reactants are frequently gaseous and the products may be liquids or liquid droplets as well as gases.
During operation of the fuel cell, hydrogen from a fuel gas stream moves from fuel channels through the porous anode electrode material and is oxidized at the anode electrocatalyst to yield electrons to the anode plate and hydrogen ions which migrate through the electrolyte membrane. At the same time, oxygen from an oxygen-containing gas stream moves from oxidant channels through the porous electrode material to combine with the hydrogen ions that have migrated through the electrolyte membrane and electrons from the cathode plate to form water. A useful current of electrons travels from the anode plate through an external circuit to the cathode plate to provide electrons for the reaction occurring at the cathode electrocatalyst.
Multiple unitary fuel cells can be stacked together to form a conventional fuel cell stack to increase the overall power output. Stacking is typically accomplished by the use of electrically conductive bipolar plates, which act both as the anode separator plate of one fuel cell and as the cathode separator plate of the next fuel cell in the stack. One side of the bipolar plate acts as an anode separator plate for one fuel cell, while the other side of the bipolar plate acts as a cathode separator plate for the next fuel cell in the stack. The bipolar plates combine the functions of anode and cathode plates referred to above and are provided with fuel channels and oxidant channels.
Fluid reactant streams are typically supplied to channels in the flow field plates via external inlet manifolds connected to the sides of the stack or by internal inlet manifolds formed by aligning openings formed in the bipolar plates and each MEA in the stack dimension. Similarly, fluid stream exhaust manifolds may be external or internal exhaust manifolds. Typically the stack also has coolant passageways extending through the bipolar plates and each MEA for circulating a coolant fluid to absorb heat generated by the fuel cell reaction.
A typical conventional bipolar flow field plate has a plurality of parallel open-faced oxidant channels on one side and a plurality of parallel open-faced fuel channels on the other side. The oxidant channels extend between an oxidant inlet manifold opening and an oxidant outlet manifold opening in the bipolar plate and typically traverse the central area of one plate surface in a plurality of passes, that is, in a serpentine manner, between the inlet manifold opening and the outlet manifold opening. Similarly, the fuel channels extend between a fuel inlet manifold opening and a fuel outlet manifold opening in the bipolar flow field plate and traverse the central area of the other plate surface in a similar plurality of passes between the fuel inlet manifold opening and the fuel outlet manifold opening.
The design of the fluid flow channel patterns is important for correct and efficient fuel cell operation. The flow channels should be sized correctly to provide the reactant species to the MEA layer, and the flow path should provide sufficient pressure drop for the flow velocities to be maintained in operation. It is desirable to be able to create flow fields with substantially different geometries on either side of the bipolar flow field plate. While serpentine channel patterns are a preferred design, channel patterns other than serpentine may be used.
The MEA is physically supported in a conventional fuel cell stack, as it is exposed to the compression forces to prevent fluid leaks between adjacent fluid flow channels in the fuel cell stack. In conventional fuel cell designs such leakage is undesirable, particularly if the channels are serpentine, as some of the fluid may move directly from the inlet manifold opening across the channels to the outlet manifold opening without passing through the passes of the channels and so missing most of the MEA. To prevent such fluid movement, flow field plates with extremely flat surfaces are required in conventional fuel cell stacks, necessitating either the grinding of the surfaces of the flow field plates or the use of molds to form the flow field plates to exacting specific tolerances.
Watkins et al. U.S. Pat. Nos. 4,988,583 and 5,108,849, issued Jan. 29, 1991 and Apr. 28, 1992, respectively, describe fluid flow field plates in which continuous open-faced fluid flow channels formed in the surface of the plate traverse the central area of the plate surface in a plurality of passes, that is, in a serpentine manner, between an inlet manifold opening and an outlet manifold opening formed in the plate.
A conventional bipolar plate is fabricated from bulk materials such as graphite and formed by creating a substantially flat plate of a given thickness and then removing materials to form flow channels in the plate. Alternately, the plate can be molded or built-up by depositing layers of materials onto a planar substrate until the desired flow channels are created. Generally, however, the manufacture of the bipolar plate remains a difficult and somewhat expensive process.
Bipolar plates manufactured according to conventional means should be made thick enough to withstand the rigors of manufacturing and be rigid enough to support the large clamping forces exercised during stack operation. This typically requires the plate to be thick enough to accommodate the full depth of the flow channels on both sides of the plate, as well as to provide a sufficiently thick solid portion in between the flow channels. This in turn means that bipolar plate design usually involves a trade-off between preferred dimensions for fuel cell operation and feasible dimensions for structural strength and cost-effective manufacturing.
It would be desirable to fabricate bipolar flow field plates for use in fuel cell stacks using low cost materials and simple manufacturing techniques that do not rely on precise machining of components. Furthermore, it would be desirable to reduce the volume of bulk material used in the formation of a bipolar flow field plate to reduce the weight and thickness of the plate, thereby improving the power density of the resulting fuel cell stack.
The use of metals for the fabrication of bipolar plates has generated considerable interest in the fuel cell design community. Metallic plates offer advantages in terms of their relative strength, high electrical conductivity and the possibility of forming shapes from thin sheets rather than hogging out thick plates or otherwise molding bulk materials. The use of metals is known to be problematic in PEM type fuel cells, however, due to corrosion and catalyst/membrane poisoning. However, the use of stainless steel 316L has resulted in moderate cell life and forms of metal coatings promise to alleviate these concerns.
Corrugated metal bipolar flow field plates have been used in molten carbonate fuel cells (MCFCs). Published Japanese Patent Publication Nos. 02-096655, 62-047968 and 59-217955 are typical examples which are constructed as a composite of two or more metallic structures formed from flat sheet materials. These designs exploit the use of multiple metallic plates formed from thin sheets and are bonded together to form an overall composite bipolar flow field plate, often with secondary channels and interstices for the storage of catalyst or other materials beneficial to the operation of molten carbonate fuel cells.
Metal bipolar flow field plates have been proposed for use in PEM type fuel cells for example, as described in U.S. Pat. Nos. 5,683,828 and 5,643,690, and Japanese Patent Publication No. 09-063601. As with their MCFC counterparts, these plates are constructed from multiple (usually at least three) individually formed plates which are subsequently bonded together to form the required flow channels.
Plates in which a single thin sheet is formed into a bipolar flow field plate are described in Japanese Patent Publication Nos. 61-128469 and 09-063599, and U.S. Pat. No. 4,755,272. These designs are advantageous in their use of simply formed corrugated metallic structures. However, these designs provide anode and cathode side flow fields that are mirror images of one another, generally with highly parallel multiple flow-paths extending from inlet to outlet plenums.
An improved flow field plate assembly supplies a reactant fluid to an electrode in a fuel cell. The assembly is suitable for use in a PEM type fuel cell stack having a series of alternating separator plate assemblies and MEA layers stacked in a stack dimension. The major components of the flow field plate assembly are: a corrugated flow field plate having corrugations that form, on each surface of the flow field plate, a plurality of open-faced fluid flow channels and a plurality of lands alternating with and parallel to the fluid flow channels; and a plurality of fluid flow couplings located in the vicinity of at least one end of the fluid flow channels for coupling a pair of adjacent flow channels on one surface of the flow field plate for flow of a first fluid between the two channels. Each coupling has a depth less than the thickness of the flow field plate such that the coupling couples flow channels on one surface of the flow field plate, yet allows a second fluid to flow by the coupling through a flow channel on the other surface of the flow field plate.
Fluids (for example, reactant or coolant) may be supplied to and removed from the flow field plate assembly by way of orifices spaced around the edges of the flow field plate. The orifices may align with orifices in an MEA layer and other flow field plate assemblies to form part of an internal manifold system of an assembled fuel cell stack.
In an assembled fuel cell stack, reactant fluids may flow on each side of the flow field plate assembly through respective flow field patterns formed by the combination of the couplings and flow channels. Oxidant flowing on one side of the flow field plate will react with fuel flowing through the MEA from the flow field of a flow field plate assembly contacting the other side of the MEA, in an electrochemical reaction that generates electricity, heat, and by-product water.
A corrugated flow field plate may be simply formed by bending a flat sheet to form channels in for example, a square wave profile; the corrugated shape may also be formed by casting, extrusion, or less preferably, by machining. Referring to one side of a corrugated flow field plate (first side), a square wave shape may form a series of parallel flow channels alternating with land surfaces in a dimension transverse to the flow channel dimension. The lands provide surfaces that physically and electrically contact an adjacent MEA. Directly opposite each land on the first side of the corrugated plate is a corresponding flow channel on the opposite side of the corrugated plate (second side), and directly opposite each flow channel on the first side of the corrugated plate is a corresponding land on the opposite second side. Thus, a first reactant fluid, for example fuel, flowing on one side of the corrugated plate is separated from a second reactant fluid, for example oxidant, flowing on the opposite side of the corrugated plate. This corrugated design tends to result in plates that are thinner than conventional bipolar flow field plates that have flow channels on one surface of the plate that do not overlap in the stack dimension with flow channels on the opposite surface of the plate. In the present approach, the flow channels on opposite sides of the corrugated layer are xe2x80x9cco-planarxe2x80x9d in the sense that the flow channels on the first and second surfaces are not in distinct layers, but instead overlap in the stack dimension at least to some degree.
The flow channels in a corrugated plate are not limited to a square wave cross-sectional shape. Also, different aspect ratios between the land and channel widths may be provided that result in larger cross-sectional channel areas on one side of the corrugated plate than the other.
In one aspect of the present corrugated flow field plate assembly, the flow channel couplings are integrally formed on the surface of the corrugated flow field plate as a depression of a portion of a land. By depressing the land to, say 50% of the channel height, fluid flowing in one channel adjacent the land will flow into the other channel adjacent the land. Depending on the layout of the couplings, a variety of flow field patterns can be created.
The depression of a land on a first surface of the corrugated plate of a certain depth will cause a corresponding embossment of the channel floor on the side directly opposite the depressed land portion. The magnitude of the depressions and embossments may be selected to reduce the interruption of fluid flow through the affected channels. This enables the creation of a flow field pattern on one surface of the corrugated plate in relative independence from a flow field pattern on the opposite surface of the corrugated plate.
For couplings formed as depressions in the lands that are inset from the ends of the corrugated plate, gaskets may be provided at the ends of the corrugated plate to seal the ends of the flow channels; the gaskets are preferably formed on or are an integral part of one of two gasket layers. Gasket layers sandwich the corrugated plate such that the gaskets fit into the ends of the flow channels. The gasket layers are also provided with cut outs to provide openings for the electrochemically active area of the fuel cell, and for the respective reactant supply and discharge manifolds.
In another aspect of the present corrugated flow field plate assembly, the flow couplings are not integrally formed on the corrugated plate, but are instead separate coupling subassemblies that attach to the ends of the corrugated plate. Each coupling subassembly has at least one coupling channel that connects the ends of two or more flow channel ends on one surface of the corrugated plate together in fluid communication. The coupling subassembly may comprise a pair of coupling structures that are adapted to nest with one another in the stack dimension such that for each nested pair of coupling structures, one coupling structure connects flow channels on one surface of the corrugated plate together in fluid communication, and the other coupling structure connects flow channels on the opposite second surface of the corrugated plate together in fluid communication. Preferably, a coupling subassembly is designed such that its thickness does not exceed the thickness of the corrugated flow field plate.
Like couplings that are integrally formed into the corrugated plate surface, the coupling subassemblies may be selectively located along each corrugated plate end to create a variety of flow field patterns. The selective nesting of coupling structures may also enable the independent formation of flow field patterns on each surface of the corrugated plate. This flexibility allows a designer to select different preferred flow field patterns for fuel and oxidant flows; for example, a single serpentine flow path may be formed on one side for high pressure drop operation typically desirable for oxidant flow while multiple flow paths may be formed on the other side to provide a lower pressure drop operation typically desirable for fuel flow.
As discussed above, the flow field plate assembly is advantageously relatively simple to manufacture, is readily formable with a variety of different flow field patterns on both sides of the assembly, and tends to be thinner than conventional flow field plates having comparable flow channel dimensions.